Depleted UF6 processing plant and method for processing depleted UF6

ABSTRACT

A depleted UF 6  processing plant including a first fluidized bed reactor configured to react depleted UF 6  with steam to produce UO 2 F 2  and hydrogen fluoride, a second fluidized bed reactor connected to the first fluidized bed reactor and configured to react the UO 2 F 2  with steam to produce U 3 O 8 , hydrogen fluoride and oxygen, a gas cooler configured to cool the hydrogen fluoride generated in the first and second fluidized bed reactors down to 150 to 300° C., and a fluorine fixing reactor containing granular calcium carbonate and connected to the gas cooler to receive the hydrogen fluoride cooled down to 150 to 300° C. from the gas cooler. The fluorine fixing reactor is configured to form granular calcium fluoride from the granular calcium carbonate and the hydrogen fluoride passing through the fluorine fixing reactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to Japanese PatentApplication No. 11-169443, filed Jun. 16, 1999 and U.S. patentapplication Ser. No. 09/494,346, filed Jan. 31, 2000. The contents ofthose applications are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a depleted UF₆ processing plantfor processing depleted UF₆ by converting UF₆ into U₃O₈, and a methodfor processing depleted UF₆.

[0004] 2. Discussion of the Background

[0005] The proportion of depleted UF₆ accumulated in an uraniumenrichment plant amount to nearly 90% of the UF₆ starting material, andit is mostly stored by filling in a UF₆ cylinder that is a cylindricalsealed storage vessel. However, since this substance is almostpermanently stored, there arises a management problem of maintaining thevessel with a large quantity of depleted UF₆ from corrosion over anextended period of time, as well as waste of resources and economicaldeficiencies caused by a vast amount of fluorine resources being storedin the form of UF₆.

[0006] A large amount of depleted UF₆ containing a low concentration ofU₂₃₅ is accumulated in the enrichment process of U₂₃₅ in the uraniumenrichment plant when U₂₃₅ is enriched using UF₆ produced from naturaluranium or recovered UF₆ as a starting material. To solve the problemsdescribed, the inventors of the present invention proposed a method forprocessing depleted UF₆ by converting depleted UF₆ containing a lowconcentration of U₂₃₅ into U₃O₈ by a dry vapor-phase reaction method(Japanese Unexamined Patent Publication No. 11-79749). The method forprocessing depleted UF₆ comprises: extracting anhydrous hydrogenfluoride as a by-product with concentrated sulfuric acid and separatinganhydrous hydrogen fluoride from dilute sulfuric acid by distillation;further distilling and concentrating dilute sulfuric acid to separatedilute hydrofluoric acid from concentrated sulfuric acid; recycling thisconcentrated sulfuric acid to the extraction and concentration stepwhile further distilling dilute hydrofluoric acid to separate it intoazeotropic hydrofluoric acid and water that contains a small amount ofhydrofluoric acid; and mixing azeotropic hydrofluoric acid with dilutehydrofluoric acid in the distillation and concentration step to improverecovery of hydrogen fluoride for recycling it in the nuclearfacilities.

[0007] However, two distillation columns and one concentration columnare required to regenerate hydrogen fluoride by the processing processof depleted UF₆ described above in the nuclear facilities. In recyclinghydrogen fluoride in the existing nuclear facilities, equipments relatedto the recycling should be additionally installed, resulting that supplyof anhydrous hydrogen fluoride does not match demands for it.Accordingly, hydrogen fluoride generated as a by-product when depletedUF₆ is converted into U₃O₈ is also desired to be recovered and storedsince it can be readily recycled.

[0008] The method for recovering and storing fluorine known in the artincludes forming calcium fluoride by a fixing reaction of fluorine tocalcium, followed by storage of calcium fluoride. The inventors of thepresent invention proposed a method for recovering granular calciumfluoride by allowing a solution mainly containing hydrogen fluoride tocontact granular calcium carbonate, and an equipment to be used for themethod (Japanese Unexamined Patent Publication No. 10-330113). Thisequipment comprises a storage tank for storing a solution containing 10to 60% of hydrogen fluoride, a first cooler for cooling the solutionstored in the storage tank to 0 to 5° C., a reaction tank for forming asolution containing granular calcium fluoride by adding granular calciumcarbonate to the solution at a temperature of 0 to 5° C. fed from thestorage tank, and a solid/liquid separator for separating granularcalcium fluoride from the solution containing it. This method is sodevised that fluorine is recovered with a high yield by forming calciumfluoride by cooling the reaction solution to 0 to 5° C. in the firstcooler.

[0009] However, the hydrogen fluoride gas generated as a by-product whenUF₆ is converted into U₃O₈ is once turned into an aqueous hydrogenfluoride solution containing 10 to 60% of hydrogen fluoride, in order torecover hydrogen fluoride as a by-product using the equipment disclosedin Japanese Unexamined Patent Publication No. 10-330113. The foregoingconversion process requires additional facilities to be installed. It isalso a problem in the conventional process described above that therecovery work becomes much complicated if hydrogen fluoride generated asa by-product in converting UF₆ into UF₃O₈ could not be recovered. Also,there is a drawback that calcium fluoride formed by the recovery ofhydrogen fluoride tends to be a fine powder.

SUMMARY OF THE INVENTION

[0010] According to one aspect of the present invention, a depleted UF₆processing plant includes a first fluidized bed reactor configured toreact depleted UF₆ with steam to produce UO₂F₂ and hydrogen fluoride, asecond fluidized bed reactor connected to the first fluidized bedreactor and configured to react the UO₂F₂ with steam to produce U₃O₈,hydrogen fluoride and oxygen, a gas cooler configured to cool thehydrogen fluoride generated in the first and second fluidized bedreactors down to 150 to 300° C., and a fluorine fixing reactorcontaining granular calcium carbonate and connected to the gas cooler toreceive the hydrogen fluoride cooled down to 150 to 300° C. from the gascooler. The fluorine fixing reactor is configured to form granularcalcium fluoride from the granular calcium carbonate and the hydrogenfluoride passing through the fluorine fixing reactor.

[0011] According to another aspect of the present invention, a fluorinefixing process for forming granular calcium fluoride includes mixinghydrogen fluoride formed in a dry vapor-phase reaction step or hydrogenfluoride formed in a waste liquor disposal step by liquefying off-gasesdischarged from the dry vapor-phase reaction step, a hydrofluoric acidrecovery step, and a condensing step by condensers installed in the dryvapor-phase reaction step, with the off-gases thereof and a wasteliquid, to form a mixture, and bringing the mixture to make contact withgranular calcium carbonate.

[0012] According to yet another aspect of the present invention, amethod for processing depleted UF₆ includes reacting depleted UF₆ withsteam to produce UO₂F₂ and hydrogen fluoride in a first fluidized bedreactor, reacting the UO₂F₂ with steam to produce U₃O₈, hydrogenfluoride and oxygen a second fluidized bed reactor connected to thefirst fluidized bed reactor, cooling the hydrogen fluoride generated inthe first and second fluidized bed reactors down to 150 to 300° C., andbringing the hydrogen fluoride cooled down to 150 to 300° C. to makecontact with granular calcium carbonate to from granular calciumfluoride.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] A more complete appreciation of the invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

[0014]FIG. 1 is a system diagram showing a flow of a processing methodand a processing plant according to an embodiment of the presentinvention.

DESCRIPTION OF THE EMBODIMENTS

[0015] The present invention will now be described in detail withreference to the drawing of the embodiment according to the presentinvention as shown in FIG. 1.

[0016] In FIG. 1, the depleted UF₆ processing plant according to oneembodiment of the present invention comprises: a first fluidized bedreactor 11 for forming UO₂F₂ and hydrogen fluoride by allowing depletedUF₆ to react with steam; a second fluidized bed reactor 12 for formingU₃O₈, hydrogen fluoride and oxygen by allowing UO₂F₂ to further reactwith steam; and a fluorine fixing reactor 13 for allowing hydrogenfluoride formed in the first and second fluidized bed reactors 11 and 12to have contact with calcium carbonate 13 a. The first and secondfluidized bed reactors 11 and 12 are plate type fluidized bed reactorseach having a plurality of chambers, and increase in the installationarea for the plant is suppressed by using the plate type fluidized bedreactors. The plate type fluidized bed reactors in this embodimentcomprises plural chambers divided by one or plural partition plates 11 aand 12 a vertically disposed in the fluidized bed. Gas/solid separationfilters 11 b, 12 b are provided at the upper parts of the pluralchambers divided by the partition plates 11 a and 12 a. Heaters (notshown) are provided in the first and second fluidized bed reactors 11and 12, each heater being so constructed as to enable the reactiontemperature in respective fluidized bed reactors 11 and 12 to becontrolled. Steam introduced into the first and second fluidized bedreactors 11 and 12 from their bottoms is generated in a steam generator15.

[0017] The fluorine fixing reactor 13 comprises plural slender cylindersfilled with granular calcium carbonate 13 a. A gas inlet port 13 c isprovided at the bottom of each cylinder, and a discharge part 13 d isprovided at the top of the cylinder for discharging the gas passingthrough the filled calcium carbonate 13 a. A pair of partition plates 13b and 13 b, on which a number of holes for the discharge gas to passthrough but not granular calcium carbonate 13 a are provided, areprovided at the top and bottom in the cylinder, and the calciumcarbonate 13 a is filled between this pair of the partition plates 13 band 13 b. A plurality of cylinders (not shown) are disposed in a circleand exchangeable one another, and hydrogen fluoride is made to contactthe calcium carbonate 13 a filled in two cylinders among the pluralcylinders. Actually, an inlet 13 c of the first cylinder is connected tothe first and second fluidized bed reactors 11 and 12 via a first piping14, and a gas cooler 16 is provided at the first piping. A dischargeport 13 d of the first cylinder is connected to an inlet port 13 c ofthe second cylinder, and a discharge port 13 d of the second cylinder isconnected to a condenser 17 via a second piping 18. The fluorine fixingreactor 13 is so constructed as to form granular calcium fluoride byallowing gaseous hydrogen fluoride generated in the first and secondfluidized bed reactors 11 and 12 to sequentially contact the granularcalcium carbonate 13 a filled in the first and second cylinders. Thecylinders are arranged so that additional hydrogen fluoride is fed byreplacing saturated cylinders with fresh cylinders.

[0018] Calcium carbonate filled in the fluorine fixing reactorpreferably has a grain size of 350 to 800 μm. When the grain size isless than 350 μm, hydrogen fluoride flow is inhibited while, when thegrain size exceeds 800 μm, the total surface area of calcium carbonatediminishes, thereby reducing the amount of calcium fluoride formed.

[0019] The method for processing depleted UF₆ according to oneembodiment of the present invention using the plant having theconstruction as described above will be described below.

[0020] The method for processing depleted UF₆ preferably includes: a dryvapor-phase reaction step for forming UO₂F₂ by allowing depleted UF₆ toreact with steam at 230 to 280° C., followed by forming U₃O₈, hydrogenfluoride and oxygen by allowing UO₂F₂ to further react with steam at 600to 700° C.; and a fluorine fixing step for forming granular calciumfluorides by allowing hydrogen fluoride generated in the dry vapor-phasereaction step to contact granular calcium carbonate at 150 to 300° C.

[0021] In the method described above, UO₂F₂ grains with a mean grainsize of 100 to 250 μm and a bulk density of 3.5 g/cm² or more, andhydrogen fluoride are formed by allowing depleted UF6 to react withsteam by adjusting the reaction temperature at 230 to 280° C.; and U₃O₈hydrogen fluoride and oxygen are formed by further allowing the UO₂F₂grains having the properties as described above to react with steam byadjusting the reaction temperature at 600° C. or more. U₃O₈ thus formedhas an approximately uniform mean grain size and an increased bulkdensity by about 10%, besides having good fluidity and being easy inhandling to improve storage efficiency.

[0022] Dry Vapor-phase Reaction Step

[0023] A reaction temperature in the first fluidized bed reactor 11 forallowing depleted UF₆ to react with steam is controlled to 230 to 280°C. with a heater (not shown), while a reaction temperature in the secondfluidized bed reactor 12 for allowing UO₂F₂ to further react with steamis controlled to 600 to 700° C. UO₂F₂ granules with a mean grain size of100 to 250 μm and a bulk density of 3.5 g/cm³, and hydrogen fluoride areformed by allowing depleted UF₆ to react with steam in the firstfluidized bed reactor 11 controlled to 230 to 280° C. When the reactiontemperature in the first fluidized bed reactor 11 is less than 230° C.,physical properties of the granules may be deteriorated, while thereaction temperature exceeding 280° C. is not desirable since the bulkdensity is decreased. Accordingly, a preferable reaction temperature inthe first fluidized bed reactor 11 is 180 to 260° C.

[0024] U₃O₈, hydrogen fluoride and oxygen are formed by allowing theUO₂F₂ granules having the properties as described above to further reactwith steam in the second fluidized bed reactor 12 controlled to 600 to700° C. The recovered UF₃O₈ powder is accommodated in a storage vessel19 for storage of an extended period of time. UF₃O₈ granules having amean grain size of 100 to 250 μm have good fluidity and easy handling,and a bulk density of 3.5 g/cm³, which improves storage efficiency. SuchUF₃O₈ granules can be obtained by processing UF₆ as described above. Areaction temperature of the second fluidized bed reactor 11 of less than600° C. is not desirable since the reaction ratio may decrease while theequipments may be corroded at a temperature over 700° C. Accordingly, aparticularly preferable temperature is from 600 through 650° C.

[0025] Fluorine Fixing Process

[0026] Hydrogen fluoride as by-products of the first fluidized bedreactor 11 and second fluidized bed reactor 12 is introduced into thefluorine fixing reactor 13 via the first piping 14. A temperature ofhydrogen fluoride passing through the first piping is controlled to 150to 300° C. with the gas cooler 16. The hydrogen fluoride controlled to150 to 300° C. flows from the gas inlet 13 c of the first and secondcylinders into the bottom partition plate 13 b and advances throughcalcium carbonate granules. Calcium fluoride is formed by allowing thehydrogen fluoride to react with the calcium carbonate granules 13 a,thereby trapping the hydrogen fluoride. A part of the hydrogen fluoridemay be condensed when the reaction temperature is less than 150° C.,while a temperature of over 300° C. is not desirable since a grain sizeof resultant calcium fluoride becomes too fine. Accordingly, apreferable reaction temperature for hydrogen fluoride is from 200through 250° C. The gas after trapping hydrogen fluoride is dischargedfrom the discharge port 13 d by passing through the upper partitionplates 13 b in the second cylinder.

[0027] The gas discharged from the discharge port 13 d is transferred tothe condenser 17 through the second piping 18. A dilute aqueous hydrogenfluoride solution liquefied and recovered in the condenser 17 istemporarily received in a storage vessel 17 a for utilizing itthereafter. Since the content of fluorine fractions in the aqueoushydrogen fluoride solution received in the storage vessel 17 a is sosmall that corrosion of the steam generator 15 is negligible exceptsmall influence on the reaction characteristics in the first and secondfluidized bed reactors 11 and 12. Therefore, the aqueous hydrogenfluoride solution can be used for generating steam for the dryvapor-phase reaction process after being transferred to the steamgenerator 15.

EXAMPLES Example 1

[0028] Hydrogen fluoride was processed in the fluorine fixing reactor13. 10 kg each of calcium carbonate granules 13 a with a grain size of350 μm were filled into the first and second cylinders. After adjustingthe temperature of the hydrogen fluoride at 200° C. with the gas cooler,fluorine was fixed by feeding the gas at a feed rate of 2000litters/hour. A hydrogen fluoride gas discharged from the secondcylinder was recovered in the condenser 17 at a recovery rate of 2.5litters/hour as a dilute aqueous hydrogen fluoride solution, and theconcentration of fluorine in the solution was measured to be 800 ppm.Feed of the hydrogen fluoride was stopped after 30 minutes, and calciumfluoride formed by conversion of calcium carbonate was recovered.

Comparative Example 1

[0029] Hydrogen fluoride was processed by the equipment disclosed inJapanese Unexamined Patent Publication No. 10-330113 described in therelated art. 1.3 kg of calcium carbonate granules were added to 1 litterof an aqueous solution containing 50% of hydrogen fluoride whileadjusting the temperature at 30° C. After a solution containing calciumfluoride granules had been formed, calcium fluoride was recovered bysolid/liquid separation.

[0030] Evaluation

[0031] The purity of the recovered calcium fluoride, the conversionratios and purity of the hydrogen fluoride, the concentrations of theaqueous hydrogen fluoride solution condensed in the condenser, and thepurity of the calcium fluoride granules were determined with respect toExample 1 and Comparative Example 1. The results are shown in TABLE 1.TABLE 1 EXAMPLE 1 COMPARATIVE EXAPLE 1 PURITY OF CALCIUM FLUORIDE 97% ORMORE 97% OR MORE CONVERSION RATIO OF FLUORINE 99% OR MORE 90% OR MORECONCENTRATION OF AQUEOUS HYDROGEN 800 PPM 5% BY WEIGHT FLUORIDE AFTERTHE REACTION PROPORTION OF CALCIUM FLUORIDE LESS THAN 5% LESS THAN 15%GRANULES HAVING A PARTICLE SIZE OF 100 μm OR LESS

[0032] Table 1 clearly shows that, although relatively high puritycalcium fluoride is obtained in both Example 1 and Comparative Example1, fluorine is converted into calcium fluoride with a higher conversionratio in Example 1 than in Comparative example 1, because hydrogenfluoride is directly introduced into the fluorine fixing reactor in agaseous state.

[0033] Since the concentration of the aqueous hydrogen fluoride solutionin Example 1 after the reaction is 800 ppm, corrosion of the steamgenerator is negligible even when the dilute aqueous hydrogen fluoridesolution is used as a steam source in the dry vapor-phase reactionprocess, leaving no influence on the reaction characteristics of thefirst and second fluidized bed reactors 11 and 12.

[0034] Moreover, while the proportion of the calcium fluoride particleshaving a grain size of 100 μm or less formed after forming the finepowders was less than 15% in Comparative example 1, the correspondingproportion in Example 1 was less than 5%. Therefore, formation of a finepowder is more suppressed in Example 1 than in Comparative Example 1.

[0035] One aspect of the present invention is to provide a fluorinefixing reactor for forming granular calcium fluoride by allowinghydrogen fluoride cooled with a gas cooler to make contact with granularcalcium carbonate, thereby enabling granular calcium fluoride to beformed by allowing gaseous hydrogen fluoride to contact calciumcarbonate after directly introducing gaseous hydrogen fluoride into thefluorine fixing reactor. Therefore, needs for the equipments required inthe conventional process such as a distillation column and concentrationcolumn, or a converter for converting gaseous hydrogen fluoride into anaqueous solution of hydrogen fluoride and a storage tank for storing thesolution, are eliminated, making it possible to simplify a depleted UF₆processing plant.

[0036] Another aspect of the present invention is to provide a gascooler for cooling hydrogen fluoride generated in the first and secondfluidized bed reactors at a temperature of 150 to 300° C., thus enablingthe hydrogen fluoride generated in the first and second fluidized bedreactors to be directly introduced into the fluorine fixing reactor.Accordingly, a depleted UF₆ processing process is simplified besidespreventing calcium fluoride particles from collapsing by allowing thehydrogen fluoride to contact the calcium carbonate at a temperature of150 to 300° C. Consequently, the granular shapes of the calcium fluorideare maintained, making handling of the calcium fluoride for fixingfluorine easy in the following processing steps.

[0037] Efflux of the secondary waste water is reduced by using thedilute aqueous hydrogen fluoride solution generated in the fluorinefixing step for steam in the dry vapor-phase reaction step, therebyreducing the processing plant size and enabling depleted UF₆ to becheaply processed.

[0038] Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A depleted UF₆ processing plant comprising: afirst fluidized bed reactor configured to react depleted UF₆ with steamto produce UO₂F₂ and hydrogen fluoride; a second fluidized bed reactorconnected to the first fluidized bed reactor and configured to react theUO₂F₂ with steam to produce U₃O₈, hydrogen fluoride and oxygen; a gascooler configured to cool the hydrogen fluoride generated in the firstand second fluidized bed reactors down to 150 to 300° C.; and a fluorinefixing reactor containing granular calcium carbonate and connected tothe gas cooler to receive the hydrogen fluoride cooled down to 150 to300° C. from the gas cooler, the fluorine fixing reactor configured toform granular calcium fluoride from the granular calcium carbonate andthe hydrogen fluoride passing through the fluorine fixing reactor.
 2. Adepleted UF₆ processing plant according to claim 1, further comprisingrecovering means for recovering the hydrofluoric acid generated in thefirst and second fluidized bed reactors, wherein anhydrous hydrogenfluoride produced as a by-product is extracted with sulfuric acid andseparated from the sulfuric acid by distillation, the sulfuric acid isfurther distilled and concentrated to separate dilute hydrofluoric acidfrom the sulfuric acid, the sulfuric acid thus concentrated is recycledfor extraction and the distillation, the dilute hydrofluoric acid isdistilled to separate into azeotropic hydrofluoric acid and watercontaining a trace amount of hydrofluoric acid, the azeotropichydrofluoric acid is fed back to the anhydrous hydrogen fluoride, andthe water containing a trace amount of hydrofluoric is reacted withgranular calcium carbonate to form granular calcium fluoride.
 3. Afluorine fixing process for forming granular calcium fluoride,comprising: mixing hydrogen fluoride formed in a dry vapor-phasereaction or hydrogen fluoride formed in waste liquor disposal byliquefying off-gases discharged from the dry vapor-phase reaction,hydrofluoric acid recovery, and condensing by condensers installed inthe dry vapor-phase reaction, with the off-gases thereof and a wasteliquid, to form a mixture; and bringing the mixture to make contact withgranular calcium carbonate.
 4. A depleted UF₆ processing plant accordingto claim 1, wherein the depleted UF₆ processing plant comprises a dilutehydrofluoric acid discharge type processing plant.
 5. A method forprocessing depleted UF₆, comprising: reacting depleted UF₆ with steam toproduce UO₂F₂ and hydrogen fluoride in a first fluidized bed reactor;reacting the UO₂F₂ with steam to produce U₃O₈, hydrogen fluoride andoxygen a second fluidized bed reactor connected to the first fluidizedbed reactor; cooling the hydrogen fluoride generated in the first andsecond fluidized bed reactors down to 150 to 300° C.; and bringing thehydrogen fluoride cooled down to 150 to 300° C. to make contact withgranular calcium carbonate to from granular calcium fluoride.
 6. Amethod for processing depleted UF₆ according to claim 5, furthercomprising: extracting anhydrous hydrogen fluoride produced as aby-product during a dry vapor-phrase reaction of depleted UF₆ withsulfuric acid; distilling the hydrogen fluoride with the sulfuric acid;further distilling and concentrating the sulfuric acid to separatedilute hydrofluoric acid from the sulfuric acid; recycling the sulfuricacid thus concentrated for extraction and the distillation; distillingthe dilute hydrofluoric acid to separate into azeotropic hydrofluoricacid and water containing a trace amount of hydrofluoric acid; feedingthe azeotropic hydrofluoric acid back to the extracting step to producethe anhydrous hydrogen fluoride; and bringing the water containing atrace amount of hydrofluoric to react with granular calcium carbonate toform granular calcium fluoride.